1. Field of the Invention
The invention relates in general to an apparatus for driving a fluorescent lamp, and more particularly to an apparatus for driving a fluorescent lamp by dynamically adjusting the driving voltage.
2. Description of the Related Art
With the improvement and innovation of science and technology, the development of display technology changes rapidly and makes progress at a tremendous pace. The traditional CRT (Cathode Ray Tube) display has gradually dropped out of the display market due to its large volume and serious radiation and is gradually replaced by LCD (Liquid Crystal Display) monitors. An LCD monitor includes fluorescent lamps for backlighting. Cold-cathode fluorescent lamps (CCFL) are commonly used as back-light due to the durability and high efficiency.
A sufficiently high startup AC voltage is required to start up a cold-cathode fluorescent lamp, and then an operation voltage which is much lower than the startup voltage is needed to make the lamp be lighted. For example, the startup AC voltage for a 15xe2x80x3 LCD monitor is 1200V, and the operation voltage is only 600V. In practice, the voltage source of the LCD monitor is usually a DC voltage of 12V, and the startup voltage and the operation voltage are generated thereby.
FIG. 1 is a block diagram showing a traditional apparatus for driving a fluorescent lamp. A DC-AC inverter is needed to transform the DC 12V into AC 1200V because the startup voltage needed by the fluorescent lamp to start up is 1200V, and the power voltage is only DC 12V. A Royer type inverter is commonly used. An AC 1200V is generated by the inverter 120 according to the DC voltage source of 12V. It is well known that the instance that, as the capacitor C1 is charged by a voltage source, the impedance of the capacitor C1 changes. According to this transient state, the voltage of AC 1200V generated by the inverter 120 is applied to the fluorescent lamp 130 at start up. Then the capacitor C1 reaches a stable state and the voltage across the fluorescent lamp 130 is designed to decreased to 600V, which is the operating or operation voltage.
FIG. 2 is a diagram of time vs. the voltage of the fluorescent lamp. At first, a startup voltage of 1200V is applied to the fluorescent lamp 130 because the impedance of the capacitor C1 is zero at the transient state. Then, an operation voltage of 600V is applied because the capacitor C1 reaches the stable state.
However, the driving voltage outputted by the inverter 120 is 1200V regardless of the voltage demand of the fluorescent lamp. While the operation voltage is only 600V, the inverter still outputs 1200V. There are some disadvantages. For example, the power efficiency is bad, heat is generated more, and bodily harm may be caused. In addition, the power consumption for a notebook is more critical. The traditional apparatus for driving the fluorescent lamp causes much power waste and needs to be further improved.
Moreover, the fluorescent lamp degrades with time, and needs a higher startup voltage. For example, a new fluorescent lamp needs a startup voltage of 1200V, and but after a few years it may need the a startup voltage of 1800V. The traditional approach to solve this problem is to set the startup voltage to a voltage higher than needed, such as 1800V, to ensure that a few years later the fluorescent lamp is still workable. This approach causes much more power waste.
The disadvantages of the traditional apparatus for driving the fluorescent lamp are as follows:
1. Bodily harm may be caused because the output voltage of the inverter remains at a very high level.
2. Power is wasted due to the high output voltage of the inverter.
3. The insulation material should be good enough, which costs more.
It is therefore an object of the invention to provide an improved apparatus for driving the fluorescent lamp by dynamically changing the driving voltage to save power and reduce the insulation requirement.
The invention achieves the above-identified objects by providing a new apparatus for driving a fluorescent lamp. The apparatus includes a dynamic driving voltage generator and an inverter. The dynamic driving voltage generator is coupled to a DC voltage source for outputting a dynamic driving voltage. The inverter is coupled to the dynamic driving voltage generator and the fluorescent lamp for outputting a lamp-driving voltage according to the dynamic driving voltage. Wherein, the lamp-driving voltage is used to drive the fluorescent lamp, the lamp-driving voltage is fed back to the dynamic driving voltage generator, and the dynamic driving voltage generator outputs the driving voltage according to the lamp-driving voltage.
Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings.